The invention relates generally to motor drive converters and more particularly to three-level neutral-point-clamped converters.
Conventional converters include electrical switches paired with diodes to accommodate inductive motor load currents. Three phase three level inverters have three phase legs with each leg having four switch and diode pairs spanning a DC bus and two clamping diodes. A controller is used for controlling each of the switches, and there are three command states for the switches per phase. By coupling clamping diodes between the DC capacitor bank midpoint (the neutral point) and pairs of the switches, the maximum DC working voltage across any switch is kept from exceeding about one half of the DC bus voltage.
Conventional techniques for providing voltage balance for the neutral point include, for example, sine triangle modulation by inserting zero-sequence voltage and space vector modulation by manipulating small vectors. Examples of these techniques are described in Steinke, xe2x80x9cSwitching Frequency Optimal PWM Control of a Three-Level Inverter,xe2x80x9d IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol. 7, No. 3, 487-496, July 1992, and commonly assigned Lyons et al., U.S. Pat. No. 5,910,892. In such embodiments, the voltage balance in the neutral point can be achieved in a line cycle level. However, for certain loading conditions, significant charge flows in and out of the neutral point and can cause neutral point voltage ripple on multiples of the line frequency.
The conventional method for minimizing neutral point voltage ripple by increasing the DC link capacitance is expensive. It would therefore be desirable to minimize neutral point voltage ripple by a method that does not require increased DC link capacitance.
Briefly, in accordance with one neutral point drift compensation embodiment of the present invention, a high power motor drive converter comprises: a three level neutral point clamped (NPC) output power conversion stage including switches for supplying power to an AC drive motor; a split series connected DC capacitor bank coupled in parallel with the NPC output power conversion stage; and a controller for selecting switch positions for controlling the NPC output power conversion stage and compensating for a neutral point voltage imbalance of the DC capacitor bank by adjusting amplitudes of carrier voltages according to an amount of voltage imbalance in the split series connected DC capacitor bank.